The present invention relates to a heat-developable photosensitive element and, more particularly, to an element capable of providing images having good image discrimination, enhanced image density and accelerated silver development.
It is well known that various cleavage reactions are assisted by silver ions including reactions involving cleavage of a compound into one or more fragments. U.S. Pat. No. 3,719,489 discloses silver ion assisted cleavage reactions useful in photographic systems. As disclosed therein, photographically inert compounds are capable of undergoing cleavage in the presence of silver ions made available imagewise during processing of a silver halide emulsion to liberate a reagent, such as a photographically active reagent or a dye in an imagewise distribution corresponding to that of said silver ions. In one embodiment disclosed therein, color images are produced by using as the photographically inert compounds, color providing compounds which are substantially non-diffusible in the photographic processing composition but capable of undergoing cleavage in the presence of the imagewise distribution of silver ions and/or soluble silver complex made available in the undeveloped and partially developed areas of a silver halide emulsion as a function of development to liberate a more mobile and diffusible color-providing moiety in an imagewise distribution corresponding to the imagewise distribution of said ions and/or said complex. The subsequent formation of a color image is the result of the differential in diffusibility between the parent compound and liberated color-providing moiety whereby the imagewise distribution of the more diffusible color-providing moiety released in the undeveloped and partially developed areas is free to transfer.
Color-providing compounds useful in the above process form the subject matter of U.S. Pat. No. 4,098,783, a continuation in part of said U.S. Pat. No. 3,719,489. The color-providing compounds disclosed therein may comprise one or more dye radicals and one or more 1,3-sulfur-nitrogen moieties. For example, they may comprise one complete dye or dye intermediate and one cyclic 1,3-sulfur-nitrogen moiety. Alternatively, the color-providing compounds may comprise two or more cyclic moieties for each dye radical or dye intermediate or vice versa.
Heat-developable photosensitive imaging materials are well known in the art, including thermally developable black and white as well as color photosensitive materials. Further, it is known in the art that such imaging materials may include various image dye-providing materials to provide the desired image. For example, Japanese Kokai 59-180548 having a Laid-Open date of Oct. 13, 1984 discloses a heat-developable silver halide photosensitive imaging system wherein the dye-providing material contains a heterocyclic ring containing a nitrogen atom and a sulfur or selenium atom which heterocyclic ring is subject to cleavage in the presence of silver ions to release a diffusible dye. An example of a suitable dye-providing material is a thiazolidine dye such as disclosed in U.S. Pat. No. 4,098,783. The process involves imagewise exposing the photosensitive system to light and subsequently or simultaneously heating the photosensitive system, in the presence of a base or base precursor, under a substantially water-free condition whereby an oxidation-reduction reaction between the exposed photosensitive silver halide and a reducing agent occurs. In the exposed areas, a negative silver image is formed. In the unexposed areas, the silver ion, present in inverse proportion to the silver image, causes the heterocyclic ring of the dye-providing material to be cleaved, releasing a diffusible dye. The diffusible dye is then transferred to an image-receiving layer, whereby a positive dye image is formed.
Generally speaking, a heat developable photosensitive system useful in terms of thermal development of the silver halide latent image is one which comprises a support carrying photosensitive silver halide, a silver salt oxidizer, a thermal solvent, a reducing agent for the silver salt, a binder and an image dye-providing material. Prior art disclosures teach the desirability of having the silver salt and the silver halide in the same layer within the photothermographic element. U.S. Pat. No. 4,452,883 teaches that the photographic silver halide material must be in catalytic proximity to the light insensitive silver source (the silver salt) and refers to catalytic proximity in terms of intimate physical association of these materials. U.S. Pat. No. 4,483,914 discloses that the silver halide and the silver salt oxidizing agent which form the starting point for development should be present within a substantially effective distance of one another and for this purpose should be present within the same layer.
However, difficulties have been encountered with such heat-developable photosensitive systems wherein image discrimination is based on soluble silver ions and/or soluble silver complex because of prerelease of dye from mobile silver during coating or during photothermal processing before silver development takes place. Such prerelease of dye is a major contributor to higher-than-desired Dmin values.
As the state of the art advances, novel approaches continue to be sought in order to attain the required performance criteria for these systems. The present invention relates to a photothermographic image-recording element.